binary tree vertical order traversal

1def verticalOrder(self, root):
2    cols = collections.defaultdict(list)
3    queue = [(root, 0)]
4    for node, i in queue:
5        if node:
6            cols[i].append(node.val)
7            queue += (node.left, i - 1), (node.right, i + 1)
8    return [cols[i] for i in sorted(cols)]